Most organisms respond to stimuli in their environment by movement (taxis) toward or away from the stimulus; yet the molecular mechanism of taxis is poorly understood in any organism. The research proposed here will explore bacterial behavior, especially phototaxis (movement toward light) and chemotaxis (movement toward or away from chemicals) in Rhodospirillum rubrum and chemotaxis in Bacillus subtilis. The ultimate objective is to describe the behavior in molecular terms. More immediately, we ask such questions as the following: How are attractants or repellents recognized? How do changing concentrations of chemicals or intensities of light affect swimming behavior? How do ions and drugs that affect motility act? What is the mechanism by which the phototactic and chemotactic information is conveyed to the flagella and translated into action? In what way is photosynthesis required for phototaxis? We will attempt to answer these questions by analyzing the effect of various chemicals on behavior, some in cells rendered permeable to the external medium, and by isolating and characterizing mutants. Applications of this work are conceivable for understanding cancer and wound healing, processes where cells' sensing their environment are important features, and abnormal behavior since some of the same drugs and hormones that affect human beings also cause behavioral change in bacteria.